Sequence stratigraphy of the lower-middle Hueco transition interval (lower Permian, Wolfcampian), Robledo Mountains, New Mexico

A 96-m-thick, mixed siliciclastic-carbonate interval spanning the boundary between the lower and middle members of the lower Permian (Wolfcampian) Hueco Formation in the southeastern Robledo Mountains, New Mexico, contains depth-sensitive lithofacies that allow delineation of sea-level cycles on the Robledo Shelf. Carbonate lithofacies include (1) fenestral dolomudstone (supratidal to high intertidal), (2) peloidal dolomudstone (low intertidal to lagoon), (3) foram packstone and grainstone (restricted marine), (4) intraclast, fossiliferous grainstone (tidal channel), and (5) fossiliferous packstone (open marine), whereas (6) shale (offshore marine) constitutes the only siliciclastic lithofacies. The presence of both siliciclastic and carbonate lithofacies may be related to sea-level change but may also have been influenced by paleoclimate, with carbonate sediment deposited during more arid periods and siliciclastic sediment deposited during more humid periods. Compared to the remainder of the Hueco Formation, the Robledo Shelf experienced more restricted marine conditions, was shallower, and was not traversed by rivers during deposition of the lower-middle Hueco transition. Several scales of sea-level cyclicity are present within the lower-middle Hueco transition, including decimeter-to meter-scale upwardshallowing parasequences, decimeter- to meter-scale sequences primarily represented by interbeds of carbonate and shale, and meter- to dekameter-scale composite megasequences composed of deeper-water sets of sequences overlain by shallower-water sets of sequences. A rough estimate of the average duration of the sequences is about 87 k.y., which is consistent with a glacial-eustatic origin. The average duration of the composite megasequences (about 600 k.y.), however, is longer than the longest glacial-eustatic cycle (about 400 k.y.), suggesting the possible role of non glacial eustacy and/or tectonic subsidence as controlling factors.